This invention relates to a method of bonding together rubber particles into a coherent rubber composition using a urethane or other bonding material. In particular, the invention relates to the use of improved pre-coating agents which promote bonding action between the bonding material and the rubber particles. The invention further relates to rubber particles that have been treated with a pre-coating agent of the invention, to the coherent rubber compositions obtained as products of the method of the invention and to their use as sports surfaces or play surfaces.
A substantial industry has become established for the manufacture of rubber composition surfaces for use, for example, in children play areas (referred to herein as xe2x80x9cplay surfacesxe2x80x9d) and in surfaces on which games are played (referred to herein as xe2x80x9csports surfacesxe2x80x9d). The rubber compositions produced for use in play surfaces and sports surfaces are conventionally produced by bonding together rubber particles using a urethane prepolymer. After mixing the rubber particles and prepolymer together the resulting liquid composition is spread on a substrate, for example in a mould so as to produce a tile or on the ground or similar substrate to produce an artificial surface in situ. Hitherto, such compositions have in many instances proved unsatisfactory in not being sufficiently coherent in use, resulting in an unduly short life of the laid surface. It is a generally accepted standard in the industry that the tensile strength of the rubber composition should be not less than 0.4 N/mm2. However, it has been found that in order to attain consistently that value of tensile strength special care has to be taken in laying the composition, such as the use of high levels of bonding material.
It has previously been proposed to increase the degree of bonding between the rubber particles and polyurethane structure by coating (referred to hereinafter as xe2x80x9cpre-coatingxe2x80x9d) the rubber particles with a polyether polyol prior to mixing with the prepolymer. Although some benefit has been obtained in this way, the resulting increase in tensile strength has been slight.
It has now been found according to the present invention that a very substantial increase in tensile strength of the rubber composition product can be obtained by using particular novel pre-coating agents. The increased coherence obtained using the pre-coating agents of preferred embodiments of the invention is such as to give a marked degree of assurance that satisfactory reproducible results can be obtained.
Accordingly, in a first aspect the present invention provides a method of producing a coherent rubber composition from particles of rubber, which comprises:
(a) coating the rubber particles with a polymeric unsaturated hydrocarbon compound;
(b) mixing the resulting coated rubber particles with a bonding material; and
(c) allowing and/or causing the resulting mixture to form the desired coherent rubber composition.
In a second aspect the present invention provides rubber particles having a coating thereon of a polymeric unsaturated hydrocarbon compound.
In a third aspect the present invention provides a process for producing from particles of rubber a layer of a coherent rubber composition suitable for use as a sports or play surface, which comprises:
(a) coating the rubber particles with a polymeric unsaturated hydrocarbon compound;
(b) mixing the resulting coated rubber particles with a bonding material;
(c) spreading the resulting mixture as a layer on a substrate; and
(d) allowing and/or causing the resulting mixture to form the desired coherent rubber composition.
The process of the invention can be used, for example, to produce coherent rubber compositions either by moulding or by a wet pour procedure.
The polymeric unsaturated hydrocarbon compound used as the pre-coating agent in this invention conveniently has a long chain aliphatic structure. Thus, for example, it can be derived from a polymer of a diene or other polyene, for example a polymer of 1,3-butadiene. The molecular weight of the pre-coating agent can, for example, be in a range up to about 9,000; for example it can be 1,500, 2,000, 2,500, 3,000, 3,500 or 4,000 or other value in the range 1,500 to 4,000. Good results have been obtained using a compound having a molecular weight of about 2,800.
In a preferred form of the invention the polymeric unsaturated hydrocarbon compound has terminal or other hydroxyl substitution. In particular, excellent results have been obtained using a hydroxyl-substituted polybutadiene, for example a terminally hydroxyl-substituted polybutadiene also known as hydroxyl-terminated polybutadiene (HTPB). HTPB has the following general formula: 
where n lies in the range 10 to 25, preferably 10 to 20, for example about 17. Where n=17 the calculated molecular weight for the dihydroxy substituted polybutadiene is 2788.
A hydroxyl-substituted polybutadiene that has been found to be most effective in the present invention, as illustrated in the Examples that follow, is the material sold under the trade mark LIQUIFLEX H by Petroflex Industria and Commercio SA. That material is a HTPB. It is obtained via free radical polymerisation and is characteristically an unsaturated liquid homopolymer with a number average molecular weight of 2800 and a degree of polymerisation of about 50. Its backbone microstructure is configured similarly to that of butadiene-styrene copolymers prepared by cold emulsion polymerisation. Typical properties of LIQUIFLEX H are as follows:
The microstructure of the HTPB has three types of unsaturation, as follows:
Further data regarding HTPB is given in a brochure of Petroflex Industria and Commercio SA under the title xe2x80x9cHydroxyl-terminated polybutadiene; a versatile polyol for the elastomer industryxe2x80x9d.
In the light of the results obtained and referred to below, it is believed to be preferable to use a pre-coating agent which is a hydroxyl-substituted polyolefin having a branch chain structure, in particular a structure which contains one or more allyl groups.
With regard to the quantity in which the pre-coating agent is used, suitable amounts are, for example, 0.2% or more of the weight of the rubber particles, especially an amount in the range 0.2 to 0.7%, for example 0.5%.
The pre-coating agent can be applied to the rubber particles using any convenient procedure. However, excellent results have been obtained by mixing the pre-coating agent with the rubber particles, for example by mechanically mixing the pre-coating agent either in a single added amount or in two or more separate amounts.
The bonding material used to bond together the pre-coated rubber particles is conveniently a prepolymer having terminal xe2x80x94NCO groups such as those conventionally used as moisture-curing adhesives. In the method of the invention the prepolymer is converted into a polyurethane and/or polyurea structure by allowing or causing it to react with water (for example in the form of atmospheric water vapour) or other suitable hydroxyl-containing compounds which cause or allow the prepolymer to cure to produce a polyurethane and/or polyurea structure. Where the production of that structure is carried under controlled conditions, for example in the preparation of tiles or other moulded sheet products, the curing can be expedited by the use of elevated temperatures.
The rubber particles used in the method of the present invention can be in any suitable physical form, for example in the form of powder, granules or crumb. In general, excellent results have been obtained using the rubber in the form of crumb, for example crumb having a dimension of 2-4 mm that is crumb passing a 4 mm mesh but retained on a 2 mm mesh. The crumb is conveniently one that has been obtained by the comminution of old motor vehicle tires or other scrap rubber, or can be manufactured specifically for this use, such as the product known as SPECTRUFLEX from Rushden Playsafe Ltd.
In order to reduce the viscosity of the hydroxyl-terminated polybutadiene it has been found beneficial to use it in conjunction with a small amount of a plastisiser (for example diisodecyl phthalate) or a small amount of a solvent (for example methylene chloride). Such viscosity-reducing additives can be used, for example, in an amount of 10 to 40 especially 15 to 25% by weight of the hydroxyl-terminated polybutadiene. As shown in Table II below satisfactory results have been obtained using such additives in an amount of 20% by weight of the hydroxyl-terminated polybutadiene. Moreover, use of the additives in that amount has had no detrimental effect on the tensile strength or elongation of the resulting product.
There is now described, by way of example, a method of for the production of a coherent rubber composition using a hydroxyl-terminated polybutadiene pre-coating agent according to the first aspect of the invention. That method is described in Examples 1 to 4 below; comparative methods are described in Comparative Examples A to F.